The present invention generally relates to a bearing and a rotating machine mounting such a bearing. More specifically, the present invention is directed to a magnetic bearing suitably used when a DC brushless motor is employed as a driver, and also to a rotating machine mounting such a magnetic bearing, and further to a method for driving such a rotating machine.
In highspeed rotating machines driven by DC motors, brushless motors are usually employed in order to avoid bad influences giving to operation gas when brushes of these DC motors wear, and further so as to extend maintenance cycles of the DC motors. In a brushless motor, a close relationship between positions of magnetic poles of this brushless motor and positions of windings to be energized must be established. Since output torque of a motor is produced by mutual reaction between magnetic fluxes generated from magnetic poles of a rotor and currents of windings of a stator, in a DC brushless motor, it is desirable to supply a current to a winding of such a phase located in the vicinity of such a position where the magnetic flux density produced from the magnetic poles of the rotor is maximum. In other words, the winding supplied with current is switched time to time with relation to the rotational position of a magnetic pole of the rotor. The timing of a commutation corresponding to the pole position may constitute an important factor in order to maximize the motor torque. A rotation position sensor is employed so as to detect the magnetic pole position of the rotor. A drive signal is produced based upon the rotation position information acquired from this rotation position sensor in such a manner that an in-phase relationship can be established between a basic wave of a motor current and a induced voltage. Then, the DC brushless motor is drived by this drive current.
As previously explained, the information about the rotation position is required in order to find the current commutation timing. The rotation positions of the rotor can be detected from the induction voltages produced by rotating of the permanent magnet rotor, so that there is no need to especially provide such a rotation position sensor. In accordance with this method, there are many advantages that a rotary system of a motor can be made compact and can be manufactured in low cost. Accordingly, various ideas have been proposed. These ideas are described in JP-A-9-266690, JP-A-9-56192, JP-A-9-294391, and JP-A-10-23783.
As related applications filed in the United States, there are applications filed on Feb. 13, 1998 entitled "Two-stage centrifugal compressor", and another application Ser. No. 08/921,604 filed Sep. 2, 1998 entitled "Multi-stage compressor" for disclosing mechanical structures of compressor.
To support rotors of a high speed turbo compressor, magnetic bearings are used because of such a merit that no lubricating oil is needed. In magnetic bearing type turbo compressors, rotors are supported in such a manner that the rotors are floated in air by receiving electromagnetic force. Moreover, active type magnetic bearings for generating electromagnetic force has means for detecting a displacement of a rotor along a radial direction to control the rotor position. The rotor is controlled using the detected radial displacements with so-called PID control. The PID control is a combination of proportional control, integral control and differential control. As explained above, in the case that magnetic bearings are used for rotating machines, a vibration of rotors should be considered. That is, highspeed rotors are occasionally driven beyond several orders of critical speeds. Then it is a key point to suppress vibrations excited by imbalance for operating rotors beyond critical speeds. For this reason, the exciting force having the frequency coincident with the rotational frequency of the rotor is produced by the magnetic bearing, and this exciting force may be used as the counter force with respect to the unbalanced force, so that the highspeed rotor can be driven beyond the critical speed.
In rotating machines supported by magnetic bearings and equipped with DC brushless motors, rotation position information is required so as to find the commutation at good timing. In general, rotation position sensors such as encoders or resolvers have been used. However, since the rotation position may be detected even by employing the induced voltage, it is a recent trend to use the induced voltage instead of a rotation position sensor.
For instance, when a 120-degree energizing type DC brushless motor rotates under no current in motor, an induced voltage is generated by a permanent magnet rotation. This generated induced voltage directly appears at winding terminals of this DC brushless motor. The induced voltage is completely synchronized with the rotation of the rotor. When this induced voltage is integrated, such a signal having a phase shifted by 90 degrees from the phase of this induced voltage is obtained. When this phase shift signal is triggered at zero cross points by employing a comparator, a pulse signal which is turned ON/OFF and OFF/ON every 180 degrees is produced. This pulse signal can be employed as a rotation position signal. It should be understood that the above-explained energizing operation is performed with respect to each of the windings that comprises the brushless motor, namely 3 windings.
Next, a drive signal of the motor is produced in synchronism with ON/OFF of the above-described rotation position signal to drive this DC brushless motor, if the motor current is small. When a current flows through the motor, a phase of a terminal voltage is not made coincident with a phase of an induced voltage due to an inductance component of this motor. Moreover, this phase difference is changed in response to the motor current. As a result, the phase of the rotation position signal is corrected in response to variations of the motor current. Thus rotation position sensors are no longer required in brushless motors. The above-described Japanese patent publications describe the concrete methods more in detail.
On the other hand, in the magnetic bearing mounted rotating machines, in particular, in the high-speed super critical rotating machines the vibrations caused by the unbalance must be suppressed in order to operate the rotating machines at speeds over the critical speed. To this end, when the magnetic bearing is employed, data corresponding to the rotation speed of the motor is required to control the magnetic bearing, and thus either the rotation position sensor or the rotation speed sensor is still employed for the magnetic bearing. When a rotation angle sensor is newly mounted on a highspeed rotor, a length of this highspeed rotor along a shaft direction thereof has to be extended, which may cause lowering a critical speed. Under such a circumstance, it is desirable to use information related to rotation position owned by a motor without newly employing a rotational speed sensor, or a rotation angle sensor. However, none of the above-described Japanese publication could satisfy this desirable aspect.